The present invention relates to a rotary braking member that performs as a braking element of a vehicle brake system, and a rust proofing process therefor. More specifically, the present invention relates a brake drum for use in a drum brake or a brake disc rotor for use in a disc brake that has a sliding surface covered with a rust preventive phosphate coating, and a rust proofing process therefor.
Motor vehicles and other vehicles are generally exported abroad by sea transportation. In the meantime, the vehicles are kept in a storage site near an export port, shipped out, kept in a storage site near an import port, and then, finally delivered to users in an importing country. There may be a case where rust forms on e.g. a brake disc rotor of a disc brake in the vehicle.
The degree of rust formation varies depending on e.g. whether a portion of the brake disc rotor is covered with a brake pad or not. If the brake is operated in such a rusting condition, there arises a variance of braking torque. This phenomenon is so-called xe2x80x9cjudderxe2x80x9d, which undesirably causes a vehicle occupant to have an unpleasant feel of braking.
Some rust proofing processes have been tried to prevent the formation of rust that can become a cause of xe2x80x9cjudderxe2x80x9d. In one process, a sliding surface of the brake disc rotor is subjected to dacrodized treatment. In another process, rust preventive oil is applied to a sliding surface of the brake disc rotor. Instill another process, a phosphate conversion coating is formed on a sliding surface of the brake disc rotor as disclosed in Japanese Patent Publication No. 1-58372.
It is necessary, in either of these rust proofing processes, not only to protect against rust but also satisfy basic requirements for the braking element as follows.
(a) To attain adequate rust preventive performance for the sliding surface until the vehicle is delivered to a user, without any adverse effect on braking function after the delivery.
(b) To minimize a deterioration in the friction coefficient of sliding surface under initial braking.
(c) To minimize the number of braking operations effected to recover a braking force in what is called xe2x80x9crunning-inxe2x80x9d before the vehicle is delivered to a user.
(d) To prevent wheel nuts from working loose owing to a deterioration in the friction coefficient of road wheel mounting plane.
However, the dacrodized treatment for the sliding surface of the brake disc rotor includes many treatment steps and requires high energy cost for rust proofing, thereby resulting in cost increase. In addition, the dacrodized treatment uses chromium so that much consideration must be given to the environment. Further, the application of rust preventive oil to the sliding surface of the brake disc rotor cannot achieve rust preventive performance over an extended period of time, although its oil application process is easy to perform.
In the formation of phosphate conversion coating e.g. zinc phosphate conversion coating on the sliding surface of the brake disc rotor by phosphate conversion treatment as disclosed in Japanese Patent Publication No. 1-58372, the coating needs to be 4 xcexcm or more in thickness so as to attain necessary and sufficient rust preventive performance, and both of treatment time and treatment temperature are increased for such a thick coating. As a result, the upsizing of equipment and the deterioration of energy efficiency are unavoidable.
Moreover, the built-up of sludge in chemical treatment bath inevitably occurs in the conversion treatment. It is therefore necessary to perform the additional maintenance step of removing the sludge and to dispose of the sludge as industrial waste by professional at a special plant, so that overall cost is unavoidably increased.
In order to solve the above-mentioned problems, the present invention has been made to provide a rotary braking member for use in a vehicle brake system, such as a brake drum or a brake disc rotor, which can insure sufficient rust preventive performance by means of a thin phosphate coating while satisfying the above-mentioned requirements for braking element, and a rust proofing process therefor.
According to the subject matter of Claim 1, there is provided a rotary braking member for use in a vehicle brake system, the braking member being formed into a disc or drum shape and made of an iron material and having at least a portion to be used as a sliding surface against an opposing frictional member, such as a brake pad or a brake shoe, covered with a phosphate coating, the phosphate coating being formed by electrolysis of a phosphate coating solution that contains phosphate ions, zinc ions and nitrate ions and having a thickness of 1 to 10 xcexcm, a mass of 4 to 33 g/m2 and a grain size of 50 xcexcm or less.
According to the subject matter of Claim 2 that methodologically corresponds to the subject matter of Claim 1, there is provided a rust proofing process for a rotary braking member, the braking member being formed into a disc or drum shape and made of an iron material, the process comprising the step of forming a phosphate coating on at least a portion of the braking member to be used as a sliding surface against an opposing frictional member by electrolysis of a phosphate coating solution containing phosphate ions, zinc ions and nitrate ions in such a manner that the phosphate coating has a thickness of 1 to 10 xcexcm, a mass of 4 to 33 g/m2 and a grain size of 50 xcexcm or less.
Herein, the rotary braking member refers to e.g. a cast-iron brake drum of a drum brake, or a cast-iron brake disc rotor of a disc brake.
Further, the phosphate coating solution is prepared by the use of e.g. xe2x80x9cPB-EL950Mxe2x80x9d available from Nihon Parkerizing Co., Ltd. with a phosphate ion content of 20 to 70 g/l, a zinc ion content of 20 to 50 g/l and a nitrate ion content of 30 to 80 g/l.
The electrolysis of the above-mentioned phosphate coating solution leads to the formation of dense phosphate coating i.e. zinc phosphate coating. This makes it possible to improve rust preventive performance dramatically as compared with that of the earlier technology and thereby retard the formation of rust remarkably even under extreme conditions. The dense phosphate coating attains necessary and sufficient rust preventive performance when the coating is at least 1 xcexcm in thickness. The thickness of the phosphate coating may be increased, but the improvement in rust preventive performance becomes saturated when the thickness of the phosphate coating reaches about 10 xcexcm. Thus, the thickness of the phosphate coating is adjusted to a maximum of about 8 xcexcm, preferably about 2 to 8 xcexcm, in view of rust preventive performance in the present invention.
According to the subject matter of Claim 3 that is based on the subject matter of Claim 2, the electrolysis is characteristically performed by setting the rotary braking member to a cathode in the phosphate coating solution and using as an anode an electrode opposed to the portion of the braking member to be used as a sliding surface with a predetermined distance kept between the electrode and the portion of the braking member to be used as a sliding surface.
According to the subject matter of Claim 4, it is clarified that the rotary braking member of Claim 2 or 3 is a brake disc rotor of a disc brake system.
The magnitude of electrolytic effect is dependent on various parameters, such as the shapes of electrodes, the distance between the anode and the cathode, the current density, the treatment time, the concentration of coating solution, the treatment temperature and the like. Thus, each parameter is determined according to the thickness required of the phosphate coating. In the case of limiting the formation of phosphate coating to a specific area, masking is given to the rotary braking member and/or the electrodes as needed. Depending on the shape of the rotary braking member, it is not necessary to complete the phosphate coating of the above predetermined thickness in one operation. The electrolysis may be performed in several operations.
In short, the brake disc rotor or brake drum made of an iron material has at least a portion to be used as a sliding surface against an opposing frictional member coated with a phosphate coating by the electrolysis of phosphate coating solution containing phosphate ions, zinc ions and nitrate ions in such a manner that the phosphate coating has a thickness of 1 to 10 xcexcm, a mass of 4 to 35 g/m2 and a grain size of 50 xcexcm or less, according to the subject matters of Claims 1 to 4. It is therefore possible to attain dramatically improved rust preventive performance and corrosion resistance even when the coating thickness is smaller than that of the earlier technology.
According to the subject matters of Claims 2 to 4, the thickness of phosphate coating can be made smaller. This makes it possible to improve energy efficiency by shortening the treatment time and possible to achieve good cost performance and environmental friendliness without the built-up of sludge.